In recent years, electronic information devices, such as personal computers, cell phones, and personal digital assistants (PDA), as well as audio-visual electronic devices, such as video camcorders and mini-disc players, are rapidly becoming smaller, lighter in weight, and cordless. Secondary batteries having high energy density are increasingly in high demand as power sources these electronic devices. Therefore, non-aqueous electrolyte secondary batteries, having higher energy density than obtainable by conventional lead-acid batteries, nickel-cadmium storage batteries, or nickel-metal hydride storage batteries, have come into general use. Among non-aqueous electrolyte secondary batteries, lithium-ion secondary batteries, and lithium-ion polymer secondary batteries are under advanced development.
A non-aqueous electrolyte normally selected is one capable of withstanding oxidation at a positive electrode that discharges at a high potential of 3.5 to 4.0 V and also is capable of enduring a reduction at a negative electrode that charges and discharges at a potential close to that of lithium. Typically, a non-aqueous electrolyte is obtained by dissolving lithium hexafluorophosphate (LiPF6) in a mixed solvent of ethylene carbonate (EC), having a high dielectric constant, and a linear as a low viscosity solvent. Linear carbonates, include, for example, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and similar carbonates. For polymer secondary batteries, gel electrolytes that comprise these non-aqueous electrolytes are retained in polymer elements known as plasticizers.
Transition metal oxides have been used as positive electrode active materials for non-aqueous secondary batteries. These metal oxides have an average discharge potential in the range of 3.5 to 4.0 V with respect to lithium. Transition metal oxides that have been used include, for example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2), and a solid solution material (LiCoaNibMnzO2, Li(CoaNibMnc)2O4). The positive electrode active material is mixed with the conductive agent and a binder to form the positive electrode active material mixture. The positive electrode active material mixture is applied on a current collector sheet made of an aluminum foil or is compression-molded on a sealing plate made of titanium or stainless steel, to produce a positive electrode.
A carbon material capable of absorbing and desorbing lithium has been used as the negative electrode active material in these batteries. Typical carbon materials are artificial graphite, natural graphite, baked mesophase carbons made from coal pitch or petroleum pitch, non-graphitizable carbons made by further baking those baked carbons in the presence of oxygen, and non-graphitizable carbons comprising baked bodies of oxygen-containing plastics. The carbon material is mixed with a binder and the like to be used as a negative electrode material mixture. The negative electrode material mixture is applied on a current collector sheet made of a copper foil or compression-molded on a sealing plate or in a battery case, which is made of iron or nickel, to produce a negative electrode.
When a graphite material is used as the negative electrode active material, lithium is released at an average potential of about 0.2 V. Because this potential is low compared to non-graphite carbon, graphite carbon has been used in applications where high voltage and voltage flatness are desired. However, the capacity per unit volume of the graphite material is as small as 838 mAh/cm3, and this capacity cannot be expected to further increase.
Negative electrode active materials showing high capacity include simple substances such as silicon and tin and oxides of those substances, which are capable of absorbing and desorbing lithium. See, for example, Japanese Laid-Open Patent Publication No. 2001-220124. However, when these materials absorb lithium ions, the crystal structure thereof varies and the volume increases. This may cause cracking of a particle, separation of a particle from the current collector, or the like, so that materials have the disadvantage of a short charge/discharge cycle life. In particular, the cracking of the particle causes an increase in reaction between the non-aqueous electrolyte and the active material, to form a film on the particle. This causes interface resistance to increase, reducing the charge/discharge cycle life of the battery.
When the battery case has low strength, such as a prismatic case made of aluminum or iron, or an exterior component which is made of an aluminum foil having a resin film on each face thereof (i.e., an aluminum laminate sheet), the battery thickness increases due to volume expansion of the negative electrode, such that an instrument storing the battery could be damaged. In a cylindrical battery using a battery case with high strength, because the separator between a positive electrode and a negative electrode is strongly compressed due to volume expansion of the negative electrode, an electrolyte-depleting region is created between the positive electrode and the negative electrode, thereby making the battery life even shorter.
Expansion per volume of the negative electrode can be reduced by blending nickel silicide (NiSi2), zinc, cadmium or the like, which are capable of absorbing a zero or small amount of lithium, into a material capable of absorbing lithium. However, such blending is not an effective measure against the increase in volume because the amount of lithium absorbed in the entire electrode plate, i.e. charging capability, decreases.
On the other hand, there are other useful oxide materials in oxide, specifically lithium titanium oxide (Li4Ti5O12), which is a well-known material that has a characteristic of non-expansion during lithium absorbing and desorbing. But this material has a potential of 1.55V at lithium desorbing and about 610 mAh/cm3 as volumetric capacity. As an anode material, Li4Ti5O12 has a cathodic desorbing potential and smaller volumetric capacity than those of graphite. Also WOz and MoOz are well-known anode materials, but these materials have large volume changes during the lithium absorbing and desorbing reaction.
It is necessary for the active material of the negative electrode to have a the characteristic of minimal volume change during lithium absorbing and desorbing, high volumetric capacity and a high potential verses lithium metal.